By sound spatialisation is meant the restitution of the three-dimensional characteristics—azimuth, elevation and distance—of a sound source emitting a sound of given frequency and intensity.
Numerous systems and devices are known, whereof the purpose is to solve such a technical problem. However, they suggest most often only simple immersion in a sound atmosphere, without restituting truly the three-dimensional characteristics of a sound. These systems may be implemented either in a room, in which case the room is fitted with several baffles equipped with loudspeakers, or using an acoustic headset, each headphone of the headset comprising a loudspeaker.
According to a first method, so-called stereophony over two channels, for simulating the displacement of a sound source, two loudspeakers are used, one channel per loudspeaker, and the sound intensity is weighted on both corresponding channels, while distributing the power to be transmitted among both loudspeakers. It is therefore possible to move the sound source while acting on the weighting coefficient.
However, this technique has the shortcoming of placing the sound sources inside the listener's head. Moreover, the sound sources may only be moved in a single dimension instead of the whole space.
To improve the impression of immersion into a sound atmosphere, it has been suggested to use four or five loudspeakers in a room. It is notably the “Dolby Surround” device. Such a system includes three front acoustic channels and one rear acoustic channel. A central loudspeaker and two left and right loudspeakers broadcast said front channels. The central loudspeaker focuses the sound sources regardless of the listening position.
The fourth channel is broadcast by two rear baffles and corresponds to ambiance information and reverberation effects.
However, such a system does not enable to locate the sound sources precisely since the central loudspeaker focuses them regardless of the listening position. Moreover, it is not possible in such a room to transmit distinct sounds for both ears.
According to another process for simulating spatialisation effect, a series of loudspeakers oriented toward a listening point are distributed regularly in a circle, whereas each loudspeaker broadcasts a particular sound channel. This is therefore an extension of the stereophonic method over two channels. However, the spatialisation effect is effectively obtained in only one particular point of the room, so-called focal point. The listeners situated elsewhere than at this focal point can also hear the sounds, but they are subjected to acoustic illusions, which may be compared to optical illusions.
Still, a sound perceived may be reduced to a supposedly uniform acoustic pressure, at the eardrums. Thus, at the level of an ear, the only interesting variable is the acoustic pressure at the eardrum, itself depending on the acoustic pressure at the inlet of the auditive duct, on the listener's morphology. In an acoustic headset, the purpose is to reproduce this pressure, in order to reconstruct a sound.
However, for two different listeners, the acoustic pressure at the eardrum, and even at the inlet to the auditive duct resulting from the same sound source will be different. This will not prevent them, unless they have a hearing impediment, from locating the sound source correctly.
These differences between individuals and between the ears of the same person are due to morphological difference. Indeed, thee space between the listener's ears and the presence of an obstacle, i.e. the head, on the path of the acoustic wave, induce a phase-shift and a difference in intensity in an acoustic wave issued from the same sound source. Thus, the acoustic pressure at the eardrum is different between the right ear and the left ear, for the same sound source, relative to the position of said source with respect to the listener.
Diverse devices endeavour to spatialise a sound while taking into account these differences in perception between both ears. The aim is simply to broadcast the same sound, while taking into account physical phenomena causing a phase shift and a difference in intensity, in both ears. This is the binaural principle.
However, binaural techniques rely on a database gathering experimental measurements, corresponding to <<average>> morphologies. It is not possible to model the human auditive tract, notably the auricle, whose shape it too complex to take into account all the physical phenomena necessary to computing approach. Thus, the techniques used correspond to average ears and the measurements are performed on dummies. These techniques exhibit therefore the shortcoming of not being suitable to everybody.
The patent U.S. Pat. No. 6,038,330 also divulges an acoustic headset for sound spatialisation. The headset exhibits, for each headphone, loudspeakers arranged regularly on a cap-shaped surface.
Each of said loudspeakers is combined with a waveguide to direct and concentrate the sound transmitted onto the listener's auricle.
However, spectral representation of the sound transmitted by each of said loudspeakers is modified by said waveguides. Such a headset does not enable then to reconstruct sounds spatially following the Huygens-Fresnel principle.